NASA has ordered a slate of taxi flights that’ll shuttle astronauts from US soil to the International Space Station (ISS) in the future. But SpaceX’s recent Falcon 9 loss has raised new concerns about using private enterprise rockets to shuttle humans to and from the ISS. Last week, NASA’s Aerospace Safety Advisory Panel (ASAP) voiced concern over using SpaceX for such missions in its annual report, citing its fueling protocol as troublesome.

NASA has ordered eight of what it calls “crew rotation missions” from private spaceflight providers Boeing and SpaceX — four flights from each. The additional missions are part of NASA’s Commercial Crew Program (CCP).

NASA said it won’t pay either Boeing nor SpaceX until the two companies have undergone certification. Boeing has scheduled an uncrewed flight test — what it calls an Orbital Flight Test — for June 2018; and a crewed flight test is planned for August 2018. SpaceX will hold its uncrewed flight test, called Demonstration Mission 1, in November 2017; and will have its crewed test will take place in May 2018.

Both companies must complete their test flights, and demonstrate to NASA sufficient safety standards are in place before NASA will certify them for the ISS missions. The two companies are currently upgrading launch pads, control centers, firing rooms and processing facilities to support the launches. Boeing’s Starliner CST-100 will be taken up into space on United Launch Alliance’s Atlas V rocket, while SpaceX will use its own Falcon 9 to deliver its Crew Dragon to the ISS.

SpaceX’s ‘load and go’ fueling system may be too dangerous

SpaceX uses super-chilled liquid oxygen to fuel its Falcon 9 rocket for launches and landings. It must use chilled oxygen in order to fit more fuel into the fuel tanks, thereby giving the rocket enough fuel to power its landing. But temperature constraints mean that the rockets must be fueled within 30 minutes of lift-off, a process referred to as “load and go,” lest the super-chilled oxygen begins to warm and expand while on the launchpad. SpaceX has said it’s developing fueling methods to utilize the “load and go” technique for fueling crewed missions, but under NASA safety protocol, astronauts are not allowed to be anywhere near the rocket during fueling.

In December 2015, Lt. General Thomas Stafford, who serves as chairman of the ISS Advisory Board for NASA, released a statement citing his own concerns about “load and go” fueling for manned missions. “There is a unanimous, and strong, feeling by the committee that scheduling the crew to be on board the Dragon spacecraft prior to load oxidizer into the rocket is contrary to booster safety criteria that has been in place for over 50 years, both in this country and internationally,” Stafford said in a letter to NASA Associate Administrator for Human Exploration and Operations, William Gernstenmaier.

Concerns over the technique were compounded in September 2016, when SpaceX’s Falcon 9 exploded on the launchpad during fueling for a routine pre-flight engine fire test. Last week, ASAP stated its also worried about SpaceX’s fueling system. “We strongly encourage NASA top management to scrutinize this issue and ensure that any decision to accept additional risk or novel risk controls with large uncertainties is justified by the value that will be gained,” the report said. “The decision should not be unduly influenced by other secondary factors such as schedule and budget concerns.”

SpaceX’s own investigation into the Falcon 9 loss cited its fueling protocol and a fuel tank design flaw as the two main culprits of the explosion. SpaceX founder and CTO, Elon Musk, has said that in a crewed mission, the Dragon’s launch abort system would have protected the crew from the explosion.

According to ASAP’s report, NASA is conducting its own investigation into the explosion. “We believe that the focus of the investigation must not be solely to identify and fix the specific cause of this mishap. It must focus also on improving the understanding of how the system functions in the dynamic thermal environment associated with ‘load and go’ so that other previously unidentified hazards can be discovered,” the report stated. “This is not a trivial effort. Despite testing at the component and subassembly level, systems often display ‘emergent’ behavior once they are used in the actual operational environment.”

Future exploration depends on privatizing human space flight

NASA has been using Russian rockets for flights to the ISS since the US government shuttered NASA’s own shuttle program in 2011. NASA’s new contracts with local spaceflight providers in the US will give it more access to the ISS for scientific research — research about both Earth and future deep space exploration.

By awarding these ride contracts to private companies, NASA said it’ll be better able to plan for the missions ahead of time, and cuts costs associated with relying on Russian spaceflights to shuttle astronauts to and from the ISS. Those rides cost NASA about $80 million per seat.

“Awarding these missions now will provide greater stability for the future space station crew rotation schedule, as well as reduce schedule and financial uncertainty for our providers,” said Phil McAlister, director of NASA’s Commercial Spaceflight Development Division. “The ability to turn on missions as needed to meet the needs of the space station program is an important aspect of the Commercial Crew Program.”

NASA has existing transportation contracts with Boeing and SpaceX. In 2014, NASA administrator Charles Bolden awarded some $6.8 billion in contracts to develop and manufacture human space transportation systems. Boeing has been developing the Starliner CST-100, and SpaceX developed its Crew Dragon transport system. Both firms will now provide six taxi flights each. Each flight will transport four astronauts to the ISS, along with some 220 pounds of cargo and supplies.

NASA expects its astronauts will have more time to dedicate to research on topics such as long-duration human spaceflight while on the ISS. This research will help NASA plan for possible future missions to Mars, for example.

“As NASA develops the Orion spacecraft and the Space Launch System rocket for deep space missions, including the journey to Mars, NASA is turning over low-Earth orbit crew and cargo transportation to commercial companies,” the agency said in a statement. “This two-pronged approach is critical to achieve the agency’s exploration goals.”